Rotating machinery maintenance

Turbomachinery Magazine, September/October 2019,

Rotating equipment lies at the heart of most oil & gas processes. Wells, pipelines, refineries and liquefied natural gas (LNG) plants rely on turbomachinery to perform daily operations and fulfill their energy demands.

However, the presence of aging equipment, variations in process conditions and the financial penalties incurred by downtime make it important to pay close attention to maintenance. A vital first step is to understand the operating environment, the performance of a machine and expectations with regard to turnaround times and equipment lifespan. From there, sources of degradation and potential mitigation efforts can be assessed.

Repair options

Whether it is a planned or unplanned outage, equipment needs to be inspected to understand the extent of damage and potential repair options. Visual and dimensional inspections are done to assess the components in their as-found condition. A range of non-destructive testing (NDT) procedures, such as wet magnetic particle inspection (MPI), ultrasound and liquid penetrant inspection can also be used to discover defects in components invisible to the naked eye.

Repair recommendations are made depending on type of damage, as well as size and location of defects. Based on this, it is determined whether a part can be salvaged, and if so, the most effective method of repair. Basic components, such as shaft seal sleeves, split rings and keys can be easily manufactured.

However, specialized tasks, such as improving impeller or turbine blade geometry require engineering analysis. Worn or damaged components can be refurbished by localized welding. If the damage is minimal, surface restoration coatings can be used instead of welding.

Weld repairs and coating restoration can often save expense by reconditioning the existing components instead of outright replacement. Reverse engineering is another possibility. This can even be applied to major rotating or stationary components such as rotating blades, impellers or diaphragms.

Compressor refurbishment

Oil & gas compressors perform a challenging role that places considerable wear and stress on parts. Regular maintenance inspections and planned repair schedules are essential for continued reliable operation, especially for highly stressed parts, such as rotors and critical sealing components.

Due to wear (erosion, corrosion) or damage (rubs, impact from foreign objects), the impeller may not be repairable. A new impeller must be reverse engineered and manufactured to replace the original component. Modern impellers can be made from a single forging of a low alloy or stainless steel using 5-axis milling or electrical discharge machining (EDM). These processes can save many weeks of manufacturing time while delivering a more dimensionally consistent and stronger component than the original, riveted or partially welded construction.

It may be possible to refine the impeller design along with changes to the vane geometry to provide a more efficient gas path design. Combined with improvements in materials and manufacturing techniques, efficiency and reliability can be upgraded. Gas path efficiency can be similarly improved.

Process seals

Process seals are critical for compressors in oil & gas applications where process gas leakage to the atmosphere cannot be tolerated for safety and environmental reasons. Oil bushing seals and mechanical dry gas seals (DGS) are the most common types used. Oil bushing seals have been used extensively in centrifugal compressors for many decades.

There is still a lot of rotating equipment, especially in refineries, outfitted with this type of seal. They can suffer from buffer oil leakages, problems with oil quality or issues with the auxiliaries, such as oil pumps or degassing tanks. In addition, there are cases when oil bushings have locked up and affected the mechanical stability of rotors due to increased vibration. As a result, there is a trend toward upgrading oil bushing seals with mechanical dry gas seals (DGS).

However, substantial engineering effort should be done before such an upgrade is executed. But when installed, operated and serviced properly, DGS will improve reliability of the compressor and reduce operating and maintenance costs.

Upgrading turbine blades

Steam turbine blades can fall victim to a variety of damage mechanisms from high temperature creep to erosion, fatigue and stress corrosion cracking. Boosting blade reliability is paramount when repairing a steam turbine. A complete engineering reevaluation of blade design is recommended if a failure has occurred or the turbine’s operating speed has changed.

Such evaluation includes a structural and modal analysis by means of finite-element analysis (FEA) software tools. It provides an insight about stresses in the critical areas (blade root, fillets, shroud) and natural frequencies of the blade that can be excited by the passing frequencies of stationary vanes at a certain running speed. This can cause resonance.

Based on the analysis, recommendations can be made to change the blade or locking hardware material or redesign the root. Resonance issues can be solved by avoiding the speeds of concern, modifying an airfoil or a shroud redesign to shift its frequencies and increase damping.

Upgraded blading provides both efficiency and reliability improvements, which can be further enhanced by the application of protective coatings to extend their life.

Coatings for durability

Coatings can be applied for dimensional restoration or protection of base material against wear or fouling. Air spray coatings are widely used for protection against fouling and corrosion. Teflon-based coatings, for example, applied to centrifugal compressor gas path components create a non-stick protective layer on the base metal surface.

This improves the efficiency of the parts in the presence of sticky hydrocarbons. Another type of air spray coating is used for corrosion protection. This is typically an aluminum-filled metallic- ceramic coating. Due to conductivity between coating layers, one substance acts as a sacrificial anode and prevents attack on the base metal.

Hard-face coatings, such as chrome carbide, tungsten carbide and stellite are applied by the high-velocity oxygen fuel (HVOF) process. They are harder than the base material to provide protection from erosion, rubbing or fretting. Common applications include steam turbine blades, shaft seals and other surfaces where touching and rubbing to the stationary components may occur.

In addition, hard-face coatings are sometimes used for dimensional restoration. They can be applied to rotating component bores to reclaim the required interference to the shaft, shaft journals and seal areas. This saves time during the repair process by allowing the part to be reused instead of replacing it.

■ Sulzer is an independent maintenance provider for rotating machinery with a global network of specialist service centers. For more information visit: